ComboBox Empty Selection should not Generate a TextBlock as SelectionBoxItem

[gehongyan]

What does the pull request do?

Fixes #16747

As demonstrated in issue #16747, for a ComboBox, when specifying PlaceholderText and DisplayMemberBinding simultaneously, the placeholder text always disappears, regardless of whether there is a selected item or not.

This issue was introduced by #11227.

What is the current behavior?

When updating the selection box item in a ComboBox, where a DisplayMemberBinding is set leaving ItemTemplate and SelectionBoxItemTemplate null, a TextBlock will be generated as the SelectionBoxItem:

Avalonia/src/Avalonia.Controls/ComboBox.cs

Lines 474 to 484 in 2d51602

	if(ItemTemplate is null && SelectionBoxItemTemplate is null && DisplayMemberBinding is { } binding)
	{
	var template = new FuncDataTemplate<object?>((_, _) =>
	new TextBlock
	{
	[TextBlock.DataContextProperty] = item,
	[!TextBlock.TextProperty] = binding,
	});
	var text = template.Build(item);
	SelectionBoxItem = text;
	}

The code above will always generate a non-null object of TextBlock assigned to SelectionBoxItem, which causes the ObjectConverters.IsNull converter on IsVisible of the PlaceholderTextBlock in the control template of ComboBox always returns false, hence, the issue.

Avalonia/src/Avalonia.Themes.Fluent/Controls/ComboBox.xaml

Lines 77 to 84 in 2d51602

	<TextBlock x:Name="PlaceholderTextBlock"
	Grid.Column="0"
	HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
	VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
	Margin="{TemplateBinding Padding}"
	Text="{TemplateBinding PlaceholderText}"
	Foreground="{TemplateBinding PlaceholderForeground}"
	IsVisible="{TemplateBinding SelectionBoxItem, Converter={x:Static ObjectConverters.IsNull}}" />

What is the updated/expected behavior with this PR?

The placeholder should always display when none are selected.

How was the solution implemented (if it's not obvious)?

The SelectionBoxItem should be set to null (the argument item passed to UpdateSelectionBoxItem in fact, but it will be a null under this circumstance), to allow the ObjectConverters.IsNull converter to succeed in returning true to let the PlaceholderTextBlock display.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Since 11.0.0-preview8, for a ComboBox, if the DisplayMemberBinding is set and ItemTemplate and SelectionBoxItemTemplate are left null, the SelectionBoxItem will always return a TextBlock, whose Text will be the selected member, or a null inside a TextBlock when none are selected.

Since this PR, in the same scenario, when none are selected, the SelectionBoxItem will return null (the passed-in argument item itself) instead.

Obsoletions / Deprecations

None.

Fixed issues

Fixes #16747

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0051412-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[cla-avalonia]

• All contributors have signed the CLA.

[gehongyan]

@cla-avalonia agree

[rabbitism]

Should we expose an :selection-empty like pseudo class for SelectingItemsControl? it should be very helpful.

[gehongyan]

Should we expose an :selection-empty like pseudo class for SelectingItemsControl? it should be very helpful.

I am not sure. If so, I may implement it under this PR or a new one.

[MrJul]

Good catch! it's likely an oversight when DisplayMemberBinding support was implemented in #11227.

The issue reproduces only when the selection has been set to a valid item before, so a unit test would be nice.

I wanted to ask you to write that test, but I've already done so while checking the behavior of this PR.
(For some reason I could push the master branch merge to your branch, but not another commit. )
Can you please add it to ComboBoxTests?

[Fact]
public void DisplayMemberBinding_Is_Not_Applied_To_SelectionBoxItem_Without_Selection()
{
    var target = new ComboBox
    {
        DisplayMemberBinding = new Binding(),
        ItemsSource = new[] { "foo", "bar" }
    };

    target.SelectedItem = null;
    Assert.Null(target.SelectionBoxItem);

    target.SelectedItem = "foo";
    Assert.NotNull(target.SelectionBoxItem);

    target.SelectedItem = null;
    Assert.Null(target.SelectionBoxItem);
}

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0052377-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[gehongyan]

Can you please add it to ComboBoxTests?

Hi, @MrJul,

Thank you for your reply! I have appended this code snippet to the ComboBoxTests. The test passes successfully when run locally.

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0052417-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

LGTM, thanks!